The purpose of this investigation is a continuing study to define the factors responsible for the dysfunction of the heart in conditions of stress, primarily those encountered in congestive heart failure. It is suggested that this dysfunction may reside in either or both the contractile protein or the excitation contraction coupling mechanism, i.e., the Sarcolemmal Na ion - Ka ion ATPase system and the Sarcoplasmic Reticular Ca ion ATPase system. These possible sites of dysfunction will be studied in the failing dog heart in which the pulmonary artery has been banded.